How do you know if there's smoke nearby?

• Can you see it?

• Can you smell it?

What do you do if you detect smoke in the house?

• Let everybody know!

Well, a smoke alarm does exactly the same thing. First, it tells if there's smoke in the area, then it let's off a loud noise to let everybody know.

Let's see how it does this...

The cheapest types of smoke alarms are Ionization Detectors. You can buy them for about £5 - not much to save a life is it?

Inside the smoke alarm is an Ionization Chamber. This is made of two metal plates attached to a battery. Underneath the bottom plate is a small amount of Americium-241. This is a radioactive element that emits nuclear radiation as it decays. Don't worry, the type of nuclear radiation (known as alpha particles) is very safe and, as it has a half-life of 432 years, will last a long time.

The alpha particles that are emitted by the Americium-241 have a particular purpose. They ionize Oxygen and Nitrogen atoms that exist in the air in the gap between the two plates. Ionization is the process by which an electron is removed from the outer shell of an atom and allowed to wander about on its own.

As this occurs, the now positvely charged atoms (ions) are drawn to the plate with a negative voltage, while the negatively charged solitary electrons are drawn to the positively charged plate. This process creates a small electric current as ions and electrons are drawn to the plates.

So far so good.

Now, when smoke enters the Ionization chamber, the tiny smoke particles attach themselves to the Nitrogen and Oxygen ions and turn them back into atoms. This creates a change in the electric current in the chamber and triggers the smoke alarm to start the siren.

It's very important you don't dismantle your smoke alarms to have a look at all of this.

Your life could depend on your smoke alarms working!